This refactors add_reaction in python_examples.py to use the
openapi_test_function decorator and validate result with
validate_against_openapi_schema. Minor changes have been made to the
OpenAPI format data for /messages/{message_id}/reactions endpoint.
This also adds add-emoji.md to templates/zerver/api and adds
add-emoji to rest-endpoints.md (templates/zerver/help/include).
akashaviator